In order to improve occupant protection in a car it is necessary to measure the belt force of a person who is strapped in. The data is processed and is used, for example, during an accident to calculate the optimum triggering of belt tensioners and airbags.
U.S. Pat. No. 6,230,088 B1 discloses a seat belt buckle housing which is connected via a belt force measuring device to a fixing point affixed to the bodywork of a vehicle. The belt force measuring device comprises a first part which is linked to the seat belt buckle and on which a detector in the form of a magnet is located. Furthermore, a second part is provided which is linked to the fixing point. A sensor element in the form of a GMR sensor is located on the second part. The first and second parts are linked together via two springs, each of which is freely supported. The belt force results in a deformation of the spring elements, so that the distance between the detector and the sensor element changes, which is analyzed as a measurement signal for the belt force.
Such a belt force measuring device has the disadvantage that during operation of the vehicle the characteristic curve of the sensor is frequently displaced, i.e. a zero displacement occurs. Furthermore, the measurement signals also exhibit an undesired hysteresis.